If your income falls within restrictions under a double tax treaty, you should look at the following help sheets:
On these help sheets, you would read through and answer the questions that are relevant to you. Eventually you'll get to a table where you should type/write the income covered by the double tax treaty, what article it is covered by and any tax paid on it, if applicable. Upload the article to your tax return.
If tax has been paid, you should make an entry in box 21 or 22 on your SA109 form - this will include the tax paid as a rebate in your calculation.
If you require assistance in regards to adding your HS302 or HS304 form to your SA109, the help article I have provided below will assist you with this:
Please note that you should avoid adding a new page if you already have an SA109 added to your tax return, instead you should simply navigate to that page.
The claim forms specifically state, if you've included any income in that table, you should not include it on your tax return. HMRC will have record of the income because it will be on the attached HS302 or HS304 and you will not be taxed on it.